mu4e’s composer derives from Gnus’ message editor and shares most of
its keybindings. Here are some of the more useful ones (you can use the menu
to find more):
key description --- ----------- C-c C-c send message C-c C-d save to drafts and leave C-c C-k kill the message buffer (the message remains in the draft folder) C-c C-a attach a file (pro-tip: drag & drop works as well in graphical context) C-c C-; switch the context (mu4e-specific) C-S-u update mail & re-index